Method for Controlling the Reactor Admission Temperature During the Production of Methylamine

ABSTRACT

The invention relates to a process for preparing methylamines by gas-phase reaction of methanol and ammonia as starting materials at a pressure in the range from 15 to 30 bar in the presence of heterogeneous catalyst. The starting materials are vaporized in one or more heat exchangers ( 1, 2, 3 ), superheated to produce a feed gas stream and subsequently fed into a reactor ( 4 ), with the mixing of the starting materials being able to be carried out in the feed stream to one of the heat exchangers ( 1, 2, 3 ) or at any desired position in a heat exchanger ( 1, 2, 3 ). A product gas stream comprising monomethylamine, dimethylamine and trimethylamine and also reaction by-products is taken off from the reactor ( 4 ). To control the reactor inlet temperature of the starting materials to a temperature in the range from 360° C. to 370° C., all or some of the feed gas stream or the product gas stream is passed through an adjustable valve ( 5 ) in order to vary the pressure and thus the condensation temperature.

The present invention relates to a method of regulating the reactorinlet temperature in the preparation of methylamines by gas-phasereaction of methanol and ammonia in the presence of a heterogeneouscatalyst. For the reaction to form methylamines to occur, the startingmaterials have to be preheated before being fed into the reactor.

The product stream obtained in the reaction of methanol and ammonia toform methylamines comprises monomethylamine, dimethylamine andtrimethylamine together with water and unreacted ammonia and unreactedmethanol. The reaction of methanol and ammonia to form methylamines isexothermic and occurs in the presence of a heterogeneous catalyst. Tocarry out the reaction, the starting materials methanol and ammonia arepreheated before being fed into the reactor.

The reaction of methanol and ammonia to give methylamines forms moretrimethylamine than is needed. The excess trimethylamine is added to thereactor feed and reacted in the reactor in an endothermic reaction toproduce dimethylamine and monomethylamine. The preheating of thestarting materials fed in is carried out in heat exchangers locatedupstream of the reactor. Preheating can be carried out using the productgas stream obtained in the reaction, as described in JP-A2-60 045 550.For this purpose, the product gas stream is passed through the heatexchangers used for preheating. Owing to the net exothermic nature ofthe reactions, the temperature increases in the reactor from the reactorinlet to the reactor outlet. Owing to the high temperature at thereactor outlet, no further energy is required for preheating thestarting materials in steady-state operation when the product gas streamis used for heating the starting materials.

The temperatures required in the reactor are determined by technicallimitations. The reactor inlet temperature has to be high enough for thereaction to start immediately in the reactor. Furthermore, the maximumtemperature in the reactor must not be too high, since otherwisedecomposition of the methylamines to form gaseous and solid degradationproducts takes place to an increased extent. However, the maximumtemperature in the reactor has to be high enough for most of themethanol to be reacted and for the thermal equilibrium to be reached.

In methods used at present for preheating the starting materials,fluctuations in the reactor inlet temperature can occur as a result oftemperature fluctuations in the surroundings or due to different amountsof trimethylamine to be recycled. The fluctuations in the reactor inlettemperature also lead to fluctuations in the maximum temperature in thereactor. In the most unfavorable case, the maximum reactor temperatureis so high that the methylamines decompose again.

It is an object of the present invention to provide a process forpreparing methylamines from methanol and ammonia by gas-phase reaction,in which the reactor inlet temperature and/or outlet temperature are/iscontrolled within a narrow range. Furthermore, virtually no energyshould have to be introduced from the outside for preheating andsuperheating the starting materials in steady-state operation of theprocess for preparing methylamines.

We have found that this object is achieved by the reactor inlettemperature and/or outlet temperature in the process for preparingmethylamines being controlled by passing the feed gas stream through anadjustable valve before entering the reactor or passing the product gasstream through an adjustable valve after leaving the reactor.

To prepare methylamines from methanol and ammonia as starting materials,the starting materials are firstly vaporized and superheated in one ormore heat exchangers. The starting materials which have in this way beenheated to a temperature in the range from 350° C. to 450° C., preferablyin the range from 350° C. to 400° C. and particularly preferably in therange from 360° C. to 370° C., are fed at this temperature as reactorinlet temperature into a reactor.

Apart from methanol and ammonia, the reaction by-products formed in thereaction are also fed into the reactor. For this purpose, the reactionproducts are separated off from the product gas stream in anafter-treatment after the reaction.

In the reactor, the starting materials are converted intomonomethylamine, dimethylamine and trimethylamine at a pressure in therange from 15 to 30 bar in the presence of a heterogeneous catalyst. Thereaction to form methylamines from methanol and ammonia is exothermic,so that the temperature in the reactor increases. A product gas streamcomprising monomethylamine, dimethylamine and trimethylamine togetherwith unreacted methanol and unreacted ammonia and also water produced asby-product in the reaction and further by-products such as carbonmonoxide and carbon dioxide is taken off from the reactor.

Owing to the exothermic reaction, the temperature increases from thereactor inlet to the reactor outlet. To prevent the temperature in thereactor from exceeding 450° C., heat of reaction can be removed bycooling the reactor.

In the formation of the methylamines, more trimethylamine than can beutilized is normally produced. The unutilized trimethylamine is fed backinto the reaction with the feed stream. The conversion of trimethylamineinto dimethylamine is endothermic. This consumes heat of reactionliberated in the exothermic reaction of ammonia and methanol to formmethylamines. For this reason, the reactor can also be operatedadiabatically.

The heat required for starting up the reaction can be supplied byadditional heating of the starting materials.

In the process of the present invention for preparing methylamines, thegas stream is passed through a valve after vaporization and superheatingof the starting materials. The pressure of the gas stream and thus thecondensation temperature downstream of the reactor can be varied bymeans of the valve. The valve can for this purpose be installed upstreamor downstream of the reactor.

To pass heat to the starting materials, the product gas stream ispreferably conveyed in countercurrent to the feedstream in the heatexchanger or exchangers. During the transfer of heat to heat up thestarting materials, the product stream is at least partly condensed inthe heat exchangers. To avoid erosion caused by entrained droplets ofcondensate, a condensate precipitator can be installed downstream ofeach of the individual heat exchangers.

A reduction in the cross section effected in the valve and theassociated decrease in pressure of the gas stream downstream of thevalve results in a decrease in the condensation temperature of theproduct gas stream. Owing to the lower condensation temperature of theproduct gas stream, the feedstream is heated to a lesser degree. As aresult, the reactor inlet temperature decreases. If, on the other hand,the valve is opened further and the pressure downstream of the valvetherefore increases, the condensation temperature of the product gasstream also increases. In this way, the reactor inlet temperature of thestarting materials is increased.

If the heat transferred from the product gas stream is not sufficient topreheat the starting materials, steam can be added to the product gasstream, for example in the stream flowing into one of the heatexchangers. The additional steam increases the quantity of heattransferred to the starting materials.

Apart from the addition of steam, there is also the possibility of, whenthe quantity of heat transferred from the product gas stream is toohigh, taking off part of the product gas stream before it enters theheat exchangers. In this case, a substream of the product gas stream canbe branched off at any point known to those skilled in the art betweenthe heat exchangers or in the heat exchanger. The amount of product gassubstream branched off is preferably controlled by means of a valve.

A further possible way of controlling the temperature in accordance withthe solution provided by the process of the present invention forpreparing methylamine is to vary the point at which the methanol streamis mixed into the ammonia-containing feed stream. The methanol can bemixed in at any point known to those skilled in the art between theindividual heat exchangers or at any point known to those skilled in theart in a heat exchanger. However, the methanol is preferably addedbetween two heat exchangers or downstream of the last heat exchanger. Inaddition, the feed stream comprising ammonia and reaction by-productscan be preheated before the methanol is mixed in.

The valve for varying the pressure of the product gas stream ispreferably configured so that the pressure at the valve inlet is from 0to 5 bar higher than at the valve outlet. Furthermore, the valve canpreferably be adjusted in a stepless fashion.

A valve for the purposes of the present invention is a valve in which ashutoff body, for example a plate, a cone, a piston or a sphere, opens acylindrical annular cross section parallel to the flow direction bymeans of a lifting movement, a slider in which the shutoff body opens aflow cross section perpendicular to the flow direction by means ofparallel or wedge-like surfaces or a stopcock or rotary slider in whichthe shutoff body rotates about its axis perpendicular to the flowdirection and thereby opens a flow cross section. Apart from theconstruction types mentioned, a valve can also be any furtherconstruction known to those skilled in the art for the shutting off tubecross sections in which the tube cross section can be varied.

As heat exchangers for preheating the starting materials, it is possibleto use all heat exchanger types known to those skilled in the art.Suitable heat exchangers are, for example, shell-and-tube heatexchangers, coil heat exchangers or plate heat exchangers. However,preference is given to using shell-and-tube heat exchangers. The heatexchangers used can be operated in cocurrent, countercurrent orcross-current. Furthermore, any combination of cross-current,countercurrent and cocurrent known to those skilled in the art ispossible. Thus, for example, when a plurality of heat exchangers isused, at least one heat exchanger can be operated in cocurrent while theremaining heat exchangers operate in countercurrent. The preferred modeof operation of the heat exchangers for heating the starting materialsis countercurrent.

To be able to control the reactor inlet temperature, measurements aretaken at various points on the plant for preparing methylamines. Thus,for example, the flow of the ammonia-containing feed stream and the flowof the methanol stream can be measured prior to mixing the methanol intothe ammonia-containing feed stream. Furthermore, when the methanol isintroduced into the ammonia-containing feed stream at various positions,the flow of the individual methanol substreams can be measured. When theproduct stream is divided into a plurality of substreams, a flowmeasurement can be made in any substream. Finally, the amount ofadditional steam for heating the starting materials can be measured bymeans of a flow measurement.

To control the reactor inlet temperature, it is also necessary tomeasure the reactor inlet temperature. Apart from the reactor inlettemperature, it is also possible to measure the temperature of theammonia-containing feed stream, the temperature of the methanol and thetemperature at the reactor outlet. Finally, the pressure differencebetween the stream flowing into the first heat exchanger and the productstream leaving the first heat exchanger for preheating the startingmaterials can advantageously be measured.

The liquid product streams obtained in the condensate separators and theproduct stream leaving the first heat exchanger are preferably passed toa further treatment to separate off the methylamines.

The invention is illustrated below with the aid of a drawing.

FIG. 1 shows a flow diagram of a process designed according to thepresent invention for preparing methylamines.

In the process depicted in FIG. 1 for preparing methylamine, anammonia-containing feed stream forms the feed 10 to a first preheater 1.The feed 10 can comprise ammonia together with by-products obtained inthe preparation of methylamines and also excess methylamines. In theprocess depicted in FIG. 1, the feed 10 is preheated in the firstpreheater 1, subsequently mixed with a methanol stream 11 and fed asfeed stream 12 into a second preheater 2. In the following, thesubstances present in the feed stream 12 in the second preheater 2 arereferred to as starting materials. In FIG. 1, the starting materialswhich have been heated further in the second preheater 2 are conveyed asa feed stream 13 to a superheater 3. In the superheater 3, the startingmaterials are superheated to the reactor inlet temperature. The startingmaterials which have been heated in this way to the reactor inlettemperature are fed as a reactor feed 14 to a reactor 4. In the reactor4, methanol and ammonia react exothermically to form monomethylamine,dimethylamine and trimethylamine. In addition, the trimetbylamine whichis formed in large amounts in the reaction but is required in only smallamounts and is recirculated to the reactor via the feed stream isconverted into dimethylamine and monomethylamine in an endothermicreaction. However, the net effect of all the reactions occurring in thereactor 4 is exothermic. For this reason, the temperature increasesalong the reactor 4. The reaction products are taken off via a reactoroutlet 15. The product gas stream taken off via the reactor outlet 15 ispassed through a valve 5 by means of which the pressure upstream of thevalve 5 can be varied. Apart from the embodiment shown in FIG. 1, inwhich the valve 5 is located downstream of the reactor 4, the valve 5can also be located upstream of the reactor 4. A reduction in thepressure leads to the vaporization temperature of the feed gas streamdecreasing, while an increase in the pressure leads to the vaporizationtemperature of the feed gas stream increasing.

In the embodiment shown in FIG. 1, the product gas stream is conveyed incountercurrent through the superheater 3 and the second preheater 2 andalso the first preheater 1. To avoid erosion in the first preheater 1,second preheater 2 and superheater 3, a first droplet precipitator 6 islocated between the superheater 3 and the second preheater 2 and asecond droplet precipitator 7 is installed between the second preheater2 and the first preheater 1. In the droplet precipitators 6, 7, thecondensate formed in the superheater 3 and the second preheater 2 isseparated from the product gas stream. The condensate obtained in thefirst droplet precipitator 6 is passed via a condensate outlet 17 to aproduct work-up step not shown in FIG. 1. In an analogous fashion, thecondensate obtained in the second droplet precipator 7 is passed via acondensate outlet 19 to the product work-up. The product stream leavingthe first preheater 1 is likewise conveyed via a product outlet 20 tothe product work-up. In the product work-up, monomethylamine,dimethylamine and trimethylamine are separated off from the productstream. Since the reaction to produce dimethylamine forms far moretrimethylamine than is required commercially, the excess trimethylamineis recirculated to the reaction via the feed 10. Correspondingly, excessmonomethylamine is fed back into the reaction via the feed 10.

Apart from adjusting the reactor inlet temperature by adjusting thepressure via the valve 5, it is also possible to control the reactorinlet temperature by introducing steam into the product gas streamdownstream of the valve 5, into the heating medium stream 16, 18 intothe second preheater 2 or into the heating medium stream 18 into thefirst preheater 1. Apart from the introduction of steam into the heatingmedium stream 16, 18 into one of the heat exchangers 1, 2, 3, the steamcan also be introduced directly into the heating medium in one of theheat exchangers 1, 2, 3.

Apart from the methanol feed stream 11 between the first preheater 1 andthe second preheater 2, the methanol can also be introduced into thefeed 10 to the first preheater 1 or into the feed stream 13 into thesuperheater 3. or directly into the reactor feed 14. Apart fromintroduction of all of the methanol into the feed 10 to the firstpreheater 1, the feed stream 12 into the second preheater 2, the feedstream 13 into the superheater 3 or the reactor feed 14, the methanolfeed stream 11 can also be divided into individual substreams. In thiscase, the substreams can be mixed into the feed streams at variouspositions.

Apart from the embodiment shown in FIG. 1 having a first preheater 1, asecond preheater 2 and a superheater 3, it is also possible to use onlyone heat exchanger for vaporization and superheating, one heat exchangerfor vaporization and one heat exchanger for superheating. Any othernumber of heat exchangers for vaporization and superheating of the feedstream is also conceivable.

Apart from the operation of the heat exchangers 1, 2, 3 incountercurrent as shown in FIG. 1, operation in cocurrent or incross-current is also conceivable, likewise any known combination ofcountercurrent, cocurrent or cross-current.

Particularly when using only one heat exchanger for vaporization andsuperheating of the feed stream, the methanol feed stream 11 should beable to be introduced at any position on the heat exchanger.

Controlling of the pressure of the product gas stream by means of thevalve 5, the position of the methanol inflow 11 and the optionaladdition of steam to the product stream for heating the startingmaterials enables the reactor inlet temperature to be controlled to atemperature in the range from 360° C. to 370° C. Controlling of thereactor inlet temperature to a temperature in the range from 360° C. to370° C. ensures that the temperature in the reactor does not exceed 450°C. The increase in temperature in the reactor 4 is produced by the heatliberated in the exothermic reaction to form methylamines. However, partof the heat is required for the endothermic reaction of trimethylamine.To keep the reactor temperature in the range from 360° C. to 450° C.,the reactor can be operated adiabatically or heat of reaction can beremoved by cooling the reactor 4.

The activation energy necessary for the reaction is preferably suppliedby means of electric heating at the beginning of the reaction.

LIST OF REFERENCE NUMERALS

-   1 First preheater-   2 Second preheater-   3 Superheater-   4 Reactor-   5 Valve-   6 First droplet precipitator-   7 Second droplet precipitator-   10 Feed-   11 Methanol feed stream-   12 Feed stream into the second preheater 2-   13 Feed stream into the superheater 3-   14 Reactor feed-   15 Reactor outlet-   16 Heating medium stream into the second preheater 2-   17 Condensate outlet-   18 Heating medium stream into the first preheater 1-   19 Condensate outlet-   20 Product outlet

1. A process for preparing methylamines comprising gas-phase reaction ofmethanol and ammonia as starting materials at a pressure in the rangefrom 15 to 30 bar in the presence of a heterogeneous catalyst, whichfurther comprises vaporizing the starting materials in one or more heatexchangers, superheating them to form a feed gas stream and subsequentlyfeeding this into a reactor, with the starting materials either beingmixed in the feed stream to one of the heat exchangers or at any otherposition on the heat exchanger, and taking off a product gas streamcomprising monomethylamine, dimethylamine and trimethylamine and alsoreaction by-products from the reactor, wherein the reactor inlettemperature of the starting materials is controlled to a temperature inthe range from 360° C. to 370° C. by passing part or all of the feed gasstream or product gas stream through an adjustable valve in order tovary the pressure.
 2. A process as claimed in claim 1, wherein the valvecan be adjusted steplessly.
 3. A process as claimed in claim 1, whereinthe valve is installed upstream or downstream of the reactor.
 4. Aprocess as claimed in claim 1, wherein the product gas stream is usedfor vaporizing and superheating the starting materials, resulting inpartial condensation of the product gas stream.
 5. A process as claimedin claim 1, wherein a substream of the product gas stream is used forvaporizing and superheating the starting materials.
 6. A process asclaimed in claim 1, wherein the reaction by-products formed in thereaction are separated off from the product gas stream and fed back intothe reactor.
 7. A process as claimed in claim 6, wherein the ammonia andthe reaction by-products fed back into the reactor are preheated beforethe methanol is added.
 8. A process as claimed in claim 1, wherein steamis added to the product gas stream to preheat and superheat the startingmaterials.
 9. A process as claimed in, wherein the reactor is operatedadiabatically.
 10. A process as claimed in claim 1, wherein heat ofreaction is removed by cooling the reactor.
 11. A process as claimed inclaim 1, wherein, when a plurality of heat exchangers is used forvaporization and superheating of the starting materials, a dropletprecipitator for separating condensate from the product gas stream isinstalled downstream of each heat exchanger.
 12. A process as claimed inclaim 1, wherein the pressure at the inlet of the valve is from 0 to 5bar higher than at the outlet of the valve.
 13. A process as claimed inclaim 1, wherein the starting materials are heated up electrically tostart the reaction.
 14. A process as claimed in claim 1, whereinvaporization and susperheating of the starting materials is carried outin countercurrent.
 15. A process as claimed in claim 1, whereinvaporization and superheating of the starting materials is carried outin cocurrent.
 16. A process as claimed in claim 1, wherein, when aplurality of heat exchangers is used for vaporization and superheatingof the starting materials, at least one heat exchanger operates incocurrent and at least one heat exchanger operates in countercurrent.17. A process as claimed in claim 2 wherein the valve is installedupstream or downstream of the reactor.
 18. A process as claimed in claim2 wherein the product gas stream is used for vaporizing and superheatingthe starting materials, resulting in partial condensation of the productgas stream.
 19. A process as claimed in claim 3 wherein the product gasstream is used for vaporizing and superheating the starting materials,resulting in partial condensation of the product gas stream.
 20. Aprocess as claimed in claim 2 wherein a substream of the product gasstream is used for vaporizing and superheating the starting materials.